Process for the desulfurization of petroleum oil fractions

ABSTRACT

LOW SULFUR-CONTENT PETROLEUM OIL STOCKS ARE PREPARED BY CONTACTING A SULFUR-CONTAINING OIL STOCK WITH AN ALKALI METAL, PREFERABLY SODIUM, OR ALKYLI METAL ALLOY, PREFERABLY SODIUM/LEAD, TO PRODUCE AN ALKALI SULFIDE-OIL DISPERSION. THE ALKALI SULFIDE IS REMOVED FROM THE OIL BY MEANS OF A TWO-STAGE WASHING TREATMENT. THE RECOVERED SULFIDE, AFTER ADDITIONAL TREATMENT, IS THEN ELECTROLYTICALLY DECOMPOSED THEREBY FERORMING ALKALI METAL FOR REUSE IN THE DESULFURIZATION PROCESS.

Emu" H5 11 A. B. WELTY 3,785,965 PROCESS FOR THE DESULFURIZA'TION OFPETROLEUM OIL FRACTIONS Filed Oct. 28, 1971 3 Sheets-Sheet 1 Jan B LTYPROCESS FOR THE DESULFURIZATION OF PETROLEUM OIL FRACTIONS Filed 001;.28, 1971 5 Sheets-Sheet 2 F3 E E f 2 N U HY .9 A/ IL Jan H5, B. E YPROCESS FOR THE, DESULFURIZATLUN O? PETROLEUM OIL FRACTIONS Filed Oct.28, 1971 3 Sheets-Sheet '5 Fig. 3

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United States Patent 3,785,965 PROCESS FOR THE DESULFURIZATION 0FPETROLEUM OIL FRACTIONS Albert B. Welty, Jr., Westfield, N.J., assignorto Esso Research and Engineering Company Filed Oct. 28, 1971, Ser. No.193,527 Int. Cl. (310g 17/00, 19/00 U.S. Cl. 208-408 M 15 ClaimsABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of theinvention The present invention relates to a process for thedesulfurization of a sulfur-containing petroleum oil stock. Moreparticularly, the process comprises contacting an oil stock with analkali metal or an alkali metal alloy to form an alkali sulfide-oildispersion that is subsequently water-washed under predeterminedconditions of temperature and pressure to produce an essentially alkalimetal sulfide-free oil. Still more particularly, the process comprisescontacting the separated alkali metal sulfide with alkali metalpolysulfide to yield an anhydrous sulfur-lean alkali metal polysulfide.The latter is decomposed electrolytically to produce alkali metal whichis then used in the initial contacting step.

Description of the prior art In the last several years there has been anever increasing concern about air pollution. One of the objects of thisconcern is the discharge of sulfur oxides to the atmosphere when burningsulfur-containing fuels. Over a period of many years several studieshave been con ducted with the object of developing efiicient andeconomical means of reducing the sulfur content of crude petroleum oilsand other virgin hydrocarbon fractions.

To the present, the most practical desulfurization process has beenhydrogenation of sulfur-containing oils at elevated pressure andtemperature in the presence of an appropriate catalyst. The processrequires the use of hydrogen pressures ranging from about 500 to about2500 p.s.i.g. and temperatures ranging from about 650 to about 800 F.,depending on the nature of the oil to be desulfurized and the amount ofsulfur required to be removed.

The process is efiicient in the case of distillate oil feedstocks but isless efficient when used with feeds containing undistilled oil such aswhole crudes or residua. This is due to several factors. First, most ofthe sulfur in the oils is contained in high molecular weight molecules,and it is difiicult for them to diifuse through the catalyst pores tothe catalyst surface. Furthermore, once at the surface, it is difficultfor the sulfur atoms contained in the molecules to see the catalystsurface. Additionally, the feedstocks may contain large amounts ofasphaltenes which tend to form coke deposits, under the processconditions, on the catalyst surface, thereby deactivating the catalyst.Moreover, high-boiling organometallic compounds present in such stocksdecompose and deposit metals on the catalyst surface thereby diminishingthe catalyst lifetime.

The severe operating conditions employed in the process causeappreciable cracking of high-boiling oils thereby producing olefinicfragments which, themselves, consume hydrogen, thereby lowering theprocess efficiency and increasing costs.

Alternate desulfurization processes that have been employed in the pastused alkali metal dispersions, such as sodium, as desulfurizationagents. Basically the process involved contacting a hydrocarbon fractionwith a sodium dispersion, the sodium reacting with the sulfur to formdispersed sodium sulfide (Na S). However, such a process has not provento be economically advantageous, particularly for treatment ofhigh-boiling, high-sulfur content feedstocks due to (a) the high cost ofsodium, (b) the impracticability heretofore of regeneration of thesodium sulfide formed in the process, and (c) problems related toremoval of the sodium sulfide from the oil.

The cost of sodium as a reagent, if used on a oncethrough basis, isprohibitively high, and it is therefore clear that one must be able torecover the sodium from the sodium sulfide in order to provide aneconomically viable process. Much though has been given to this problem,but until now no economical process has been developed.

For example, it is impractical to electrolyze molten sodium sulfide tosodium and sulfur because of the solfides high melting point, i.e. 1740F. and the large amount of electrical energy required at thattemperature to dissociate the sodium sulfide. In addition, the sodiumand sulfur are both produced as vapors at 1740 F. and specialprecautions would have to be taken in order to keep them apart in theelectrolytic cell. The capital and operating cost of such a cell wouldprobably be prohibitive.

It is much more convenient to form the products in the liquid phase. Analternate process would involve converting the sodium sulfide to sodiumchloride by chemical means and then electrolyzing the sodium chloride.Electrolysis of sodium chloride is well known in the art. By dissolvingthe sodium chloride in a mixture of potassium and calcium chlorides thecompound can be electrolytically dissociated at temperatures of about1100" F. However, although liquid sodium is produced, the process isstill economically impractical due to the large amount of electricalenergy required to decompose the relatively stable sodium chloride, andbecause of the problem of handling the chlorine produced.

In theory, the best approach would be electrolyze an alkali metal saltthat melts at about the same temperature as used for the desulfurizationprocess and which can be electrolyzed with minimum consumption ofelectrical energy.

The alkali polysulfides, preferably sodium polysulfides, meet the aboveenumerated requirements. There are three sodium polysulfides withmelting points as follows: Nazsg (885 F.), Na S (545 F.), and Na s, (485F.). These polysulfides are mutually soluble and intermediatecompositions, having intermediate properties, can form. A eutecticoccurs at about Na s with a melting point of 450 F. Moreover, theelectrolysis of molten sodium polysulfide consumes less electricalenergy than electrolyzing either molten sodium monosulfide or moltensodium chloride. Finally, due to the low melting points of thepolysulfides, the desulfurization process can be carried out with lessheating and cooling load and at about the same temperature as thedesulfurization reaction, and hence with less heat exchange equipmentand at lower cost.

SUMMARY OF THE INVENTION In accordance with the invention, it has nowbeen discovered that an economically feasible desulfurization processvis-avis hydrosulfurization of whole crude or residual oils can beachieved and that outstanding sulfur removal can be realized.Specifically, the process involves contacting a sulfur-containingpetroleum oil stock with a desulfurization agent comprising an alkalimetal, such as lithium, sodium, potassium, rubidium and the like,preferably sodium, or an alkali metal alloy, preferably sodium-lead,thereby forming an alkali metal sulfide-oil mixture. (Hereinafter theinvention will be described with respect to sodium although it isunderstood that other alkali metals can be used.) At least a portion ofthe mixture (in the form of a dispersion of the sulfide in the oil) issubsequently contacted with water in a two-stage contacting operationwherein the dispersion is first treated with water under high pressureand temperature conditions followed by water treatment under lowtemperature and pressure conditions, the water being used preferablyconcurrently with respect to the flow of the oil. This insures virtuallycomplete removal of the sodium sulfide therefrom, while simultaneouslyproducing a highly concentrated aqueous sodium sulfide solution.

The aqueous solution may also contain other sodium compounds such ashydroxide, sulfite, thiosulfate and carbonate which may be removed bystandard techniques as described in more detail hereinbelow. The alkalimetal sulfide solution is then contacted with at least a portion ofsodium polysulfide to vaporize the water and to yield an essentiallyanhydrous sulfur-lean sodium polysulfide. The latter is subsequentlydried and at least a portion thereof electrolytically dissociated toreform sodium metal and to produce a sulfur-rich sodium polysulfide fromwhich sulfur, corresponding to the amount removed from thesulfur-containing oil, is recovered.

Any feedstock from which sulfur is desired to be removed may in theorybe used in the instant process. Thus, suitable feedstocks include wholecrudes such as Safaniyah crude (Middle East), Lagunillas crude(Venezuela), or US. crudes, residual fractions or any distillatefraction. The subject process is particularly adapted to thedesulfurization of whole crude or residua that are difiicult to treat byother methods such as hydrodesulfurization.

While the feedstock may be fed directly to the initial contacting ordesulfurization zone without pretreatment, it is desirable to desalt thefeed in order to prevent contamination of the molten polysulfidecontained in the electrolytic cell. Desalting is a well establishedprocess in the industry. A particularly preferred desalting processinvolves the addition of a small amount of water to the oil in order todissolve the salt contained therein followed by electrical coalescers.The oil is then dehydrated by conventional methods known in theindustry.

The sodium may be used as a dispersion of the pure metal or in the formof a molten alloy such as sodium/ lead or sodium/tin. When sodium/leadis the alloy, suitable atom proportions desirably comprise about 0.3-0.5sodium/0.7-0.5 lead, and when using sodium/tin, about 0.2-0.3sodium/0.80.7 tin.

While the molten alloy form of the metal facilitates dispersion of themetal in the oil, nevertheless, it suffers from several disadvantages.First, the reaction temperature must be kept above the melting point ofthe alloy since sodium will not diffuse through solid lead. The meltingpoint of the sodium/lead alloy, for example, is about 620 F. dependingupon the proportion of sodium used in the alloy. Since it is desirableto absorb the exothermic heat of reaction by using feed substantiallybelow reaction temperature, this presents an additional engineeringcomplication. On the other hand, pure sodium metal, for example, has amelting point of about 208 F. and imposes no such limitation to theprocess. Additionally, when employing the alloy, the spent lead or tinmust be separated and recycled. On the other hand, the quantity of puresodium metal added to the oil can be controlled so that the metal iscompletely consumed.

The contacting of the sodium metal or sodium metal alloy with thesulfur-containing oil is preferably carried out at temperatures andpressures sufficient to maintain the bulk of the reactants within thereaction zone in the liquid phase. However, conditions may be varied toprovide vapor phase contact. Thus, the reaction temperature willgenerally be maintained between about 450 and 750 F., preferably 600 to700 F. The reaction pressure will depend on the feed and temperatureemployed. For reduced crude fractions the pressure will range between 10and p.s.i.g., preferably 40 to 60 p.s.i.g. For whole or topped crudepressures may be raised to as high as about 500-600 p.s.i.g. in order tomaintain all or most of the oil in the liquid phase. Lower pressures canbe used where extensive desulfurization of the lower-boiling (vaporphase) fractions is not necessary.

The sodium metal reacts with the sulfur-containing oil stock, as shownin Equation 1, to yield sodium sulfide which precipitates from the oil,forming a dispersion there- In addition, organo-oxygen contained in thefeedstock is removed therefrom by reacting with the sodium metal.Furthermore, depending on the amount of water present in the feed andthe reaction conditions, varying amounts of sodium sulfite, thiosulfateand hydroxide in addition to salts of organic acids may be formed.(Typical crudes contain between about 0.1 and 0.2% organic oxygen.)Additionally, some organo-nitrogen and organo-metals are also removedfrom the oil.

The desulfurization step is conducted as a batch or continuous typeoperation, but is preferably continuous. In general, the various meanscustomarily employed in extraction processes to increase the contactarea between the oil stock and the sodium metal or sodium metal alloycan be employed. The apparatus used in the desulfurization step is of aconventional nature and can comprise a single reactor or multiplereactors equipped with (a) shed rows or other stationary devices toencourage contacting; (b) orifice mixers; (c) efiicient stirring devicessuch as mechanical agitators, jets of restricted internal diameter,turbo mixers and the like. The petroleum oil stock and the sodium metalor sodium metal alloy can be passed through one or more reactors inconcurrent, cross current or countercurrent flow, etc. It is preferablethat oxygen and water be excluded from the reaction zone; therefore, thereaction system is thoroughly purged with dry nitrogen and the feedstockdried prior to introduction into the reactor. It is understood thattrace amounts of water, i.e. less than about 0.5 wt. percent, preferablyless than about 0.1 wt. percent based on total feed, can be present inthe reactor without affecting the process efiiciency.

The resulting oil dispersion is subsequently removed from thedesulfurization zone and washed with water in order to remove the sodiumsulfide and the relatively smaller amounts of sodium hydroxide, sodiumsulfite, sodium thiosulfate, sodium carbonate and the salts of organicacids dispersed therein. It is preferable to carry out the water-washtreatment in a plurality of stages, preferably two stages, at differentconditions of temperature in each stage. Additionally, the operatingpressure in each stage must be maintained at a suflicient level tomaintain the water in substantially the liquid phase.

The water-wash operation accomplishes several objectives:

(1) removal of substantially all of the sodium sulfide and other sodiumcompounds dispersed in the oil;

(2) reduction of the heat load and cost of subsequent separation of thewater from the sodium sulfide by minimizing the amount of water requiredto eifectuate sodium sulfide removal;

(3) employment of operating conditions suflicient to preclude emulsionformation.

As will be evident in the further description of the process and in viewof the above-mentioned process objectives, it is essential to minimizethe amount of water used in the water treatment step in order tominimize the heat requirement for its subsequent removal via contactwith hot sodium polysulfide. Thus, the water washing is conducted in thefirst stage at elevated temperatures and pressures Where the solubilityof the sodium sulfide is high, thereby requiring only a minimum amountof water. Temperatures in the range of about 400500 F., preferably425-475 F., are used for this first stage. The solubility of the sodiumsulfide in the water will be in excess of about 70 wt. percent (based ontotal aqueous solution) at this temperature. The pressure in the firststage is kept at a level sufiicient to maintain the water substantiallyin the liquid phase. Thus, a pressure ranging from about 250-700p.s.i.g., preferably 330-540 p.s.i.g., is employed. An important featureof the high temperature washing operation is the minimization ofemulsion formation, particularly with the more viscous feedstocks, dueto the re moval therefrom of certain emulsifying agents such as thesodium salts of organic acids.

At these high temperature and pressure water-treating conditions, toomuch sodium sulfide-containing water solution, about 0.8 to 1.9 wt.percent based on the oil present, remains dissolved in the oil. Toreduce the remaining sodium sulfide content to an acceptable level, theoil is introduced into a second washing stage, which is operated atrelatively low temperature and pressure conditions in order to minimizeaqueous solution solubility in the oil. Organic emulsifying agentshaving now been largely removed, emulsion formation is no longer aproblem at the low temperature. Temperatures in the range of about 150to 300 F., preferably 195 to 205 F., are suitable. Higher temperatureswithin the broad range may be necessary for more viscous oils in orderto maintain their fluidity. The pressure need be only high enough tomaintain the water substantially in the liquid phase. Thus, a pressurein the range of atmospheric to about 75 p.s.i.g., preferably 5-10p.s.i.g. is employed. By use of this dual temperature Water-washtreatment, the sodium metal content of the recovered desulfurized oil isreduced to trace levels.

The physical nature of the washing and separation system is not criticaland may be of a conventional design. While cocurrent, cross-current orcountercurrent contacting within a given stage may be employed,cocurrent contacting is preferred. In general, a system similar to thatwhich is sometimes used in crude de-salting is preferred. In such asystem, the oil and water or water solution are vigorously mixed,typically in an orifice line, or other mixer. Subsequently the waterdroplets are coalesced and settled in an electric field, and thenseparated from the oil. Contact times in each of the water-treatmentstages varies from about 5 to 30 minutes and preferably to 25 minutes.

The hot concentrated sodium sulfide solution is subsequently removedfrom the wash system. As indicated above, the solution may contain othersodium compounds such as hydroxide, sulfite, thiosulfate and carbonate.These compounds can be removed by standard techniques. Thus, thecarbonate can be continuously removed as calcium carbonate by theaddition of calcium hydroxide. Other means may also be used for removingthe carbonate. For example, other compounds whose carbonates areinsoluble may be added. Or the carbonate concentration in thecirculating molten salt can be allowed to increase to a level such thatthe incremental alkali metal added with each cycle causes precipitationof solid sodium carbonate which is separated from the molten salt. Theremaining compounds are converted to sodium sulfide by adding hydrogensulfide.

Subsequently, the hot aqueous sodium sulfide solution is contacteddirectly with hot sodium polysulfide, i.e. Na s where x varies fromabout 4.4 to 4.9, preferably from about 4.5 to 4.8, most preferably fromabout 4.5 to 4.6. The polysulfide is added to the sulfide at atemperature ranging from about 700-820 F., preferably 725 to 765 F.Simultaneously the pressure on the sodium sulfide solution is released.The relative rates and compositions of the sulfide and polysulfide arecontrolled in such a way that (1) all of the water is vaporized assteam, (2) sodium polysulfide, i.e. Na S where y ranges from about 3.5to 4.3, preferably from about 3.8 to 4.1, most preferably from about 3.9to 4.0, is formed, and (3) the temperature of the resulting sodiumpolysulfide is above its melting point, i.e. a polysulfide melt.Polysulfide with less sulfur holds water too tenaceously for it to beremoved readily. Polysulfide with more sulfur tends to be hydrolyzed byWater, forming H S. A suitable final temperature for the polysulfidemelt is about 525 to 600 F., preferably 550 to 570 F.

The reaction of the sodium sulfide solution and the hot sodiumpolysulfide is rapid and .much steam is evolved as pressure is released.Several systems are available to effect reactions having thesecharacteristics. A preferred system comprises two concentric nozzlesarranged in such a manner that the sodium polysulfide contacts thesodium sulfide solution simultaneously with the pressure release,followed by a separator that allows the steam to be taken overheadalone, while the liquid sodium polysulfide so produced collects and iswithdrawn.

The sodium polysulfide melt is dried and subsequently cycled toelectrolytic cells wherein it is dissociated to form molten sodium and asulfur-rich sodium polysulfide, i.e. Na s where z ranges from about 4.5to 5.0, preferably from about 4.6 to 4.9, most preferably from about 4.7to 4.8. The sodium is then withdrawn and either alloyed with a moltenmetal such as lead or tin or introduced directly into thedesulfurization zone in undiluted form as hereinabove described.

The electrolytic cell unit will preferably comprise a sodiumion-conducting physical and electronic barrier or membrane thatseparates alkali metal on the one side from alkali metal polysulfide onthe other side. Generally, the membrane may be composed of any materialthat can function as a sodium ion-conducting separator; however,beta-alumina containing sodium oxide is preferred. Such beta-aluminawill contain sodium oxide in the general range of about It is noted thatwhen an alkali metal other than sodium is employed in the instantprocess, the oxide of the alkali metal will be admixed with beta-aluminain lieu of Na O.

The beta-alumina may be used in the pure form or doped with a smallamount of metal oxide such as MgO, Li O and the like. A detaileddiscussion of doped betaalumina is provided in an article appearing inthe Electrochemical Society Extended Abstracts-Los Angeles Meeting-Mayl0l5, 1970, entitled Ionic Conduction in Impurity Doped [i-alumina, byAtsuo Imai et al., the disclosure of which is incorporated herein byreference. Reference is also made to US. Pats. 3,488,271 to J. T. Kummeret al. and 3,475,225 to G. T. Tennenhouse. During cell operation, sodiumions migrate from the sodium polysulfide side, i.e. the anode side,through the barrier to the sodium metal side, i.e. the cathode side,Where they are neutralized by electrons. At the same time polysulfideions give up their electrons at the electron-conducting anode to formelemental sulfur that then reacts with additional polysulfie anions toform new polysulfide ions, i.e. S of greater sulfur content.

As indicated above, z will take values in the range of about 4.5 to 5.0.The S anions are continually removed from the cell in combined form withsodium, i.e. Na S The anode may comprise any suitable electronconducting-current collector such as graphite, molybdenum, titanium,chromium or aluminum that can withstand corrosive attack of the sodiumpolysulfide. The cells are arranged preferably in series electrically,so that the anode for One cell is the cathode for the one adjacent toit. The overall reaction is shown below:

cathode side anode side In other embodiments elemental sulfur is allowedto build up in the cell and the operating temperature therein ismaintained high enough so that the sulfur is continuously removedtherefrom as vapor. Additionally, the sodium/lead alloy may be formedinternally, i.e. in situ, before the molten sodium metal is withdrawnfrom the electrolytic cell by continuously feeding lead or spentsodium/lead alloy to the cathode side of the cell.

While a beta-alumina type cell has been described, any other cell thatis capable of economically decomposing sodium polysulfide into moltensodium is suflicient for the present purposes. A particular beta-aluminaelectrolytic cell and methods for the preparation of betaalumina aredescribed in such patents as US. 3,488,271 and 3,404,036 to J. T. Kummeret al., U.S. 3,368,709 to J. T. Kummer and U.S. 3,446,677 and US.3,475,225 to G. I. Tennenhouse, the disclosures of which areincorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWING A preferred embodiment of the presentinvention is shown in the accompanying drawings:

FIG. 1 is a flow diagram of the overall desulfurization process usingsodium metal in the pure form.

FIG. 2 is a flow diagram of the overall desulfurization processutilizing an alloy of sodium, i.e. sodium/lead.

FIG. 3 is a simplified scheme showing the formation of the molten sodiumwithin the electrolytic cell.

The desulfurization reactor systems used in the instant process varydepending on whether sodium or sodium alloy is used as the reactant. Thesystem using sodium will be described first, then the system usingsodium alloy.

Turning now to FIG. 1, a sulfur-containing feedstock is fed by means ofline 1 to charge pump 2 where the pressure is raised to about 500p.s.i.g. for whole crudes and distillates and 50 p.s.i.g. for residualstocks. The feed is preliminarily desalted by conventional means (notshown), prior to introduction into line 1. The feedstock will ordinarilybe a whole crude of one to three percent sulfur or a residual stock oftwo to seven percent sulfur, although distillate stocks can be used.

The oil enters heat exchanger 4 via line 3, where the temperature israised to approximately 450 F. The oil flows via line 5 to separator 6where the hydrocarbon vapors are taken overhead through line 7. Waterpresent in the feedstock is also taken overhead as steam. To facilitatedrying, a stream of dry gas can be introduced to the bottom of separator6 (not shown). The oil enters first reactor vessel 66 via line 8.

Sodium is pumped by pump 63 from sodium storage vessel 62 via lines 68and 64 through dispersing nozzle 65 into first reactor vessel 66. Nozzle65 consists of a multiplicity of nozzles (not shown) which are soarranged as to give thorough dispersion of sodium in the flowing oil.Reactor 66 contains orifice baflies 67 to promote movement of the sodiumparticles throughout the oil. The holding time in first reactor 66 isabout 5 to 30 minutes. Temperature in this first reactor rises to about625 F., depending on the amount of sulfur to be removed.

The oil stream flows via line 9 to second reactor 10 where the remainingsodium is reacted. Second reactor 10 contains bafiies 11 to promotecontinuing contact between the sodium and oil and to prevent by-passingfrom the inlet to the outlet. The temperature at the top of secondreactor 10 is about 650 F. Gas that is formed due to the increase intemperature is taken overhead through line 12 and is condensed anddepressured by conventional means (not shown). The desulfurized oilcontaining dispersed sodium sulfide mixed with lesser amounts of othersodium compounds such as sodium oxide and hydroxide, sulfite,thiosulfate and carbonate, leaves the top of second reactor 10 via line12.

The remainder of FIG. 1 will be explained below after describing thealternate reactor system for sodium alloy shown in FIG. 2.

FIG. 2 shows the reactor system for the case where sodium alloy is used.It diifers from the case where sodium alone is used primarily in thatoil is recycled to the reactor in order to prevent cooling of the alloybelow its melting point and in order to recover and recycle the spentalloy. Feed is charged through line 1, line 3, heat exchanger 4, line 5,separator 6 and line 8, analogously and at the same conditions asdescribed for the sodium case in FIG. 1. Water vapor and hydrocarbonvapors are withdrawn via line 7. The preheated, dehydrated oil in line 8is mixed with recycled desulfurized oil from pump 68. The relative feedrates are maintained so as to keep the oil at a temperature above about620 F.

The mixed oil stream enters reactor vessel 72 via line 69 and flows inan upward direction therethrough. The vessel is of such a size that theoil remains in the reaction portion below the sodium alloy distributorfor ten minutes to one hour. The sodium alloy, preferably sodium/lead orsodium/tin or a mixture of the two enters reactor 72 via disperser 74.Atom compositions of about 0.3-0.5 Na/0.7-0.5 Pb or 0.2-0.3 Na/0.8-0.7Sn are suitable.

The alloy droplets commingle with the rising oil stream, and beingheavier than the oil and of large enough size, fall downwardly throughthe rising oil stream. Baffles 73 promote contact. The spent alloycollects in the bottom of reactor 72 and passes via line 76 tothe alloystorage vessel 77. The alloy is fortified by contact with freshlyregenerated sodium entering vessel 77 via line 61. Pump 78 feeds thefortified alloy via line 75 to disperser 74.

The desulfurized oil passes into a settler zone located in the top ofreactor 72 where any small, entrained alloy particles are allowed tosettle. Baffles 71 and 79 act as collecting plates for the alloyparticles and are placed at a slight angle so that the coalesced alloycan run off the ends. The desulfurized oil/sodium sulfide dispersionleaves reactor 72 via line 13.

One particular reactor system has been described. However, the alloy andoil can flow concurrently, and any one of several well-known types ofmixers such as pump mixers, high shear mixers or paddle blade mixers canbe employed. Other settling means, in particular external, long,horizontal flow vessels or liquid cyclones can be also used.

The sodium sulfide-oil dispersion in line 13 is pumped up to a pressureof about 475-525 p.s.i.g. by pump 14, through line 15 and through cooler16 where the temperature is reduced to about 430-500 F., preferably450'- 480 F. If residuum is being processed and distillate flux stockneeds to be added in order to make a burning fuel of suitable viscosity,this flux stock is added through line 17.

Dilute aqueous sodium sulfide solution from settler 33 is discharged,through line 35 and pump 37, through line 38 where it joins the sodiumsulfide-oil dispersion in line 18 The two streams flow cocurrentlythrough orifice mixer 36 and thence via line 19 into coalescing andseparation vessel 20. The temperature therein is about 400- 500 F.,preferably 425-475 F. and the pressure is about 250-700 p.s.i.g.,preferably 330-540 p.s.i.g. In this vessel, approximately 70 to 90weight percent of the sodium sulfide and of the other sodium compoundspresent originally dispersed in the oil is removed in the aqueous phase.Here organo-sodium compounds are hydrolyzed and the resulting sodiumhydroxide is likewise removed in the water phase. Vessel 20 provides aholding time for the oil of about to 25 minutes.

The aqueous sodium sulfide layer is withdrawn from vessel 20 by way ofline 24. The oil phase, containing a reduced amount of sodium sulfide,is removed from chamber 20 by means of line 25 and subsequently passedthrough cooler 26 and line 27 where it is joined by fresh water enteringthrough line 28. The amount of water used is the amount which when addedto the sodium present in the oil gives a highly concentrated solution inthe range of about 60-80 weight percent, preferably 65-75 Weight percentas sodium sulfide.

The combined water-oil stream passes through orifice mixer 31 andthrough pressure reducing valve 32 to coalescing and separating vessel33. The temperature in this vessel ranges from about 150-300" F.,preferably 195- 205 F. and the pressure ranges from atmospheric to about75 p.s.i.g., preferably 5-10 p.s.i.g. The desulfurized feedstock has amaximum sulfur content of about 75 to 98 weight percent of that presentin the original feed, and a maximum sodium content of about 0.05-0.5weight percent. It is removed from vessel 33 via line 34. The diluteaqueous sodium sulfide solution is withdrawn via line 35 through pump 37and recycled via line 38 to orifice mixer 36 and the first stagecoalescer settling vessel 20.

The concentrated aqueous sodium sulfide solution is withdrawn fromvessel 20, via line 24. A calcium hydroxide slurry is added via line 21to react with the sodium carbonate present. Hydrogen sulfide is addedthrough line 22 to convert the sodium hydroxide, sulfite and thiosulfatepresent to sodium sulfide. The aqueous solution is introduced intoseparation vessel 71 through orifice mixer 70. Solid calcium carbonateis separated from the solution by centrifuging, filtering, settling,etc., by known means in vessel 71 and withdrawn through line 23. Aconcentrated solution of sodium sulfide is added through line 30 inorder to replace sodium subsequently lost in the purge stream takenthrough line 49. The solution is then passed via line 29 to pressurerelease valve 39 and subsequently injected into separating zone 41.

Molten sodium polysulfide (Na s where it varies from about 4.4 to 4.9)at a temperature of about 700-820 F., preferably 725-765 F., iswithdrawn from sulfur vaporrelease vessel 42 via line 44 through pump 45and transferred via line 46 to the outlet of pressure reducing valve 39where it is mixed with the aqueous sodium sulfide solution. The mixingmust occur rapidly and completely at the point where the pressure isreduced so that solid sodium sulfide which would plug the line will notform when the steam evaporates.

The hot sodium polysulfide supplies the heat required for dissolution ofwater and its vaporization and in addition supplies the sensible heat tobring the sodium polysulfide to a temperature above its melting point.The combined stream enters vessel 41 via expanded section 40. The steamgoes overhead through line 48 and the collected sodium polysulfide isremoved via line 50. The sodium sulfide from the water solution and themolten sodium polysulfide react to form Na s where y varies from about3.5 to 4.3. The temperature in vessel 41 is 525600 F., preferably 550570F. Vessel 41 contains de-entrainment packing 47 of a conventional typeto avoid entrainment of sodium polysulfide overhead with the steam.

Line 49 is used to purge a small stream of sodium polysulfide from thesystem in order to prevent buildup of impurities to an inoperable level.These dissolved impurities arise from the feed and from equipmentcorrosion as well as from the organometalliccompositions removed fromthe feed by the action of the sodium. Specifically, compounds containingcombined iron, vanadium, silica, nickel, chromium, lead and tin may formand are removed from the system via line 49.

The molten sodium polysulfide (N Z y): thus formed, is withdrawn fromvessel 41 and passed via line 51 to nitrogen stripping vessel 52.Nitrogen that has been previously dried is introduced into vessel 52 bymeans not shown and is continuously recycled via line 53 through drier54, line 55, compressor 56 and again into vessel 52. The purpose of thenitrogen stripper is to remove as much water as possible from the moltensodium polysulfide (Na S from vessel 41. Generally, H S is added to thenitrogen via line 69 or substituted for the nitrogen in order to reactwith any hydroxide or any thiosulfate or other oxygen-containingcompounds still present, the water thus formed being continuouslyremoved by the driers.

The dried molten sodium polysulfide is subsequently removed from vessel52 via line 58 and is passed directly to electrolytic cells 59. Theelectrolytic cells are of a conventional design and may comprise anycell capable of converting the sodium polysulfide to sodium metal.Preferably, the individual cell unit comprises a molten sodiumcontainingcavity and a molten sodium polysulfide-containing cavity separated fromeach other by a sodium ion-permeable membrane comprising preferably,crystalline beta-alumina, as already described.

A schematic representation of a cell unit is shown in FIG. 3. Inopeartion, electrons flow through the metal separator sheet 1 enteringthe molten sodium-containing cavity 2, wherein sodium cations combinewith the electrons and are reduced to elemental sodium that is withdrawnfrom the cavity via line 3. The beta-alumina membrane 4 acts both as amechanical separator and an electron separator of the two cavities andat the same time as a sodium ion-permeable membrane. Sodium polysulfideis introduced into cavity 5 via line 6; it is, by its nature, highlyionized into sodium cations and polysulfide anions. The latter areoxidized to elemental sulfur that reacts further to yieldsulfur-enriched polysulfide anions. The anions along with the requisitesodium cations are subsequently removed via line 7 from cavity 5 assulfur-enriched sodium polysulfide (Na S where z varies from about 4.5to 5.0). Electrons which are given up by the polysulfide anions flowthrough the metal separating sheet "8 to form a complete circuit. Thusthe anode for one cell becomes the cathode for the next. The cell anodewill comprise a solid, electron-conducting current collector such asgraphite, molybdenum, titanium, chromium, aluminum, nickel-iron alloysand other alloys and the like.

As noted above, although beta-alumina is shown as the preferredseparator, any other separator that is suflicient for the purposes maybe employed. Additionally, an alternate embodiment comprises forming andcontinually removing elemental sulfur from the cell. In practice, theelectrolytic cell 48 comprises a plurality of individual cell units inorder to provide a sufficient output of sodium. The cells, althoughoperated in parallel relative to sodium polysulfide flow are operated inseries from an electrical point of view.

About -200 cells are operated in series in order to build up the overallvoltage to about 300 600 volts. The total amount of cell area requireddepends on the amount of sodium required, and is in the range of 10 to50 square feet per pound per minute of sodium. The temperature in thecell rises to about 700-820 F., depending on the amount of cell area,current density used, the resistance of the cell elements and theircondition. Cell pressure is atmospheric or slightly above.

The composition of the sodium polysulfide leaving the electrolytic cellcan be controlled by the fiow rate and the current. The greater the flowrate the less is the increase in sulfur content; the greater the currentthe greater is the increase in sulfur content. The composition iscontrolled such that by applying a reasonable vacuum (and/ or heat ifdesired), sulfur corresponding to that which was removed from the oilcan be taken overhead.

Accordingly, the sodium polysulfide formed in the electrolytic cell ispassed via line 60 (FIG. 1) to sulfurreducing vessel 42 which ispartially evacuated, e.g. to an absolute pressure of 10 to 300 mm. Hg,preferably 50-100 mm. Hg, to vaporize some of the sulfur and reduce thesulfur content of the polysulfide so that the final polysulfidecomposition is Nags wherein at takes values from about 4.4 to 4.9,preferably 4.5 to 4.8. At one-tenth atmosphere sulfur vapor pressure,for example, the composition in equilibrium therewith is approximatelyNa S at 700 F., Nflzs g at 750 F. and Na S at 800 F.

The sulfur vapor is taken overhead through line 43 and condensed byconventional means (not shown). As indicated supra the resultingpolysulfide is then recycled to contacting Zone 41. The molten sodium issubsequently removed from cell 59 and passed via line 61 to the reactionsystem which has already been described.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will bemore clearly understood by reference to the following hypotheticalexample (refer to FIG. 1).

Crude petroleum from the Safaniyah field in Saudi Arabia is used as theprocess feedstock. Its properties are as follows:

NaCl#/1000 bbls. Bio

The crude containing 0.20 weight percent water is desalted byconventional means thereby reducing the salt content to 1.5 pounds per1000 barrels of crude. The desalted oil is subsequently dehydrated at apressure of 550 p.s.i.g. and a temperature of 450 F., thereby producingan oil containing less than about 0.1 weight percent water.

The oil is then passed to reactor 66. Sodium, after passing through adispersion device 65 comprising uniformly spaced holes, is introducedinto reactor 66 as fine droplets and at a rate of 330 pounds per minute.The reactor has ten equally-spaced baffles perpendicular to the line offlow, each with 354 diameter holes equally spaced. At the outlet of thisvessel the temperature is 623 F. and the pressure is 480 p.s.i.g. Oilfrom reactor 66 flows to reactor where sodium reaction is completed.Reactor 10 contains eight equally-spaced partial baflies sloped as shownin FIG. 1. At the top of the reactor the temperature is 650 F. and thepressure is 465 p.s.i.g. About 9.2 weight percent of the oil is takenoverhead from the reactor as vapor for further processing or todisposition to final product.

The desulfurized oil containing the sodium sulfide and other sodiumcompounds formed is pumped up to 600 p.s.i.g. and passed through acooler where the temperature is reduced to 465 F. and thence to aconventional line orifice mixer 36.

Also entering the mixer is 196 pounds per minute of water solution fromthe second stage water wash vessel 33. This solution contains 13.5weight percent of sodium sulfide and other sodium salts and 86.5 weightpercent water. The thoroughly mixed oil-water solution enters electricalcoalescer and settling vessel 20 where the pressure is 430 p.s.i.g. andthe temperature is 450 F. The electrical coalescing vessel is 8 feet indiameter and 23 feet long.

The oil from vessel 20 passes through cooler 26 where the temperature isreduced to 200 F. This cooled oil stream joins a 170' pound per minutestream of water and together pass through a conventional line orificemixer 31. The mixed stream passes through a pressure reducing valve intocoalescer-separator 33 identical in shape to vessel 20. The pressure is20 p.s.i.g. and the temperature 197 F.

The desulfurized oil leaving this separator is bright and contains 0.14weight percent sulfur and 40, 15, and 5 p.p.m. (weight) of vanadium,nickel and iron respectively. The API density is 26.2 and the viscosityis 72 SSU at F. The desulfurized oil is sent to further processing or tofinal product.

The aqueous solution from vessel 20 contains 55.5 weight percent ofsodium sulfide, 15.0 weight percent NaOH, 4.8 weight percent Na S O 0.4weight percent Na SO 1.8 weight percent Na CO and 22.5 weight percentwater. Thirty-one pounds per minute of calcium hydroxide slurrycontaining 30% calcium hydroxide is added to the aqueous solution toreact with the sodium carbonate. One hundred and twenty-six pounds perminute of hydrogen sulfide is added to the solution to neutralize thesodium hydroxide, thiosulfate and sulfite and convert them to sodiumsulfide and sodium polysulfide. The calcium carbonate precipitate isseparated from the aqueous solution by conventional means such as byliquid cyclones, settling and filtration.

The concentrated sodium sulfide solution at 450 F.

and 430 p.s.i.g. is then fortified with 65 weight percent sodium sulfidesolution and then passes to pressure reducing valve 39. The aqueoussolution passes from the valve throat immediately into a funnel-shapedsection which expands into a three foot diameter section, which in turnempties into separator vessel 41. Sodium polysulfide having an averagecomposition of Na2s4 54 at 750 F. is injected at the narrow end of thefunnel immediately beyond the point of concentrated solution pressurerelease. This incorporates the sodium mono-sulfide into the sodiumpolysulfide and supplies the heat required for steam dissolution andvaporization. The steam and newly constituted sodium polysulfide enterseparator 41. Two hundred and forty-three pounds per minute of steam aretaken overhead. The newly constituted sodium polysulfide has acomposition Na s and amounts to 7750 pounds per minute. This sodiumpolysulfide is withdrawn from the bottom of the separator and passes tothe top of drying tower 52.

Dry nitrogen at a rate of 460 cubic feet per second passes up throughthe tower and removes traces of moisture. The moisture-containingnitrogen passes overhead through line 53, through conventional molecularsieve dryers 54, and is recirculated to the bottom of the dryer vessel52 by compressor 56. Hydrogen sulfide is continuously bled into thesystem through line 69 in order to maintain a fifty percent by volume HS concentration in the circulating nitrogen. The dry molten Na Saccumulates in the bottom of dryer vessel 52 and passes to electrolyticcells 59.

The electrolytic cells are arranged in packs of 200 cells each, arrangedin series electrically and serviced by a direct current voltage of 520volts. Each cell is 10 inches square. Each pack consumes 1050 amperesand produces 13.2 pounds of sodium per minute. Twenty-five cell packsarranged in series electrically are used and produce 330 pounds perminute of sodium and 7410 pounds per minute of sodium polysulfide ofcomposition Na S Heat is generated by the cell and the temperature risestherein to 762 F.

Each cell consists of a 30-mil thick sheet of betaalumina, having acomposition 9.0 weight percent Na O- 8.5 weight percent Mg--82.5 percentA1 0 with a 40- mil thick sodium metal compartment formed by a parallel20-mil molybdenum sheet on one side and a 40-mil thick sodiumpolysulfide compartment formed by a similar molybdenum sheet on theother side. The cells are packed together back-to-back like amulti-layered sandwich. Provision is made to interrupt the electricalcircuit at the exit of each cell Where the sodium leaves. The sodium iscollected and passes through line 61 to sodium storage vessel 62. Thesodium polysulfide passes through all of the cells in parallel.

The sodium polysulfide at 762 F. passes from the electrolytic cellsthrough line 60 to sulfur release vessel 42. A vacuum is applied toreduce pressure to 55 mm. Hg absolute and 260 pounds per minute ofsulfur vapor is taken overhead, condensed and recovered in accordancewith standard practice. The sodium polysulfide remaining has acomposition NaS and amounts to 7150 pounds per minute. Its temperatureis 750 F. This hot Na S is conveyed to pump 45 and line 46 to pressurereducing valve 39, the operation of which has been described previously.

What is claimed is:

1. A process for the desulfurization of a sulfur-containing petroleumoil stock comprising introducing said oil stock into a desulfurizationzone, contacting said oil stock therein, at desulfurization conditions,with a desulfurization agent selected from the group consisting of thealkali metals and alloys thereof, thereby precipitating at least aportion of the sulfur contained in said oil stock as alkali metalsulfide and forming an oil/ alkali metal sulfide mixture, contacting atleast a portion of said mixture with water at a pressure ranging fromabout 250 to 700 p.s.i.g. and at a temperature ranging from about 400 to500 F. thereby forming an aqueous alkali metal sulfide solution and anoil of reduced sulfur content, contacting said aqueous alkali metalsulfide solution with hot alkali metal polysulfide thereby vaporizing atleast a portion of the water from said aqueous solution and forming analkali metal polysulfide of reduced sulfur content, contacting at leasta portion of said oil of reduced sulfur content with additional Water ata pressure ranging from about 0 to 70 p.s.i.g. and at a temperatureranging from about 150 to 300 F., to remove additional metal sulfidesfrom said oil and recovering a desulfurized oil.

2. The process of claim 1 wherein said petroleum oil stock is selectedfrom the group consisting of whole crude and residua.

3. The process of claim 1 wherein said desulfurization agent is sodium.

4. The process of claim 1 wherein said oil/ alkali metal mixture iscontacted with water at a pressure ranging from about 330 to 540p.s.i.g. and at a temperature ranging from about 42.5 to 475 F. therebyforming an aqueous alkali metal sulfide solution and an oil of reducedsulfur content.

5. The process of claim 4 wherein said oil of reduced sulfur content iscontacted with water at a pressure ranging from about 5 to p.s.i.g. andat a temperature ranging from about 195 to 205 F.

6. A process for the desulfurization of a sulfur-containing petroleumoil stock comprising contacting said oil stock, at desulfurizationconditions, with a desulfurization agent selected from the groupconsisting of sodium and alloys thereof, thereby precipitating at leasta portion of the sulfur contained in said oil as sodium sulfide andforming an oil/ sodium sulfide mixture, contacting at least a portion ofsaid mixture with water at a temperature ranging from about 400 to 500F. and at a pressure ranging from about 250 to 700 p.s.i.g. therebyforming an aqueous sodium sulfide solution and an oil of reduced sulfurcontent, contacting at least a portion of said oil of reduced sulfurcontent with additional water at a temperature ranging from about to 300F. and at a pressure ranging from about 0 to 75 p.s.i.g. and recoveringa desulfurized oil, contacting at least a portion of said aqueous sodiumsulfide solution with hot sodium polysulfide thereby vaporizing at leasta portion of the Water from said aqueous sodium sulfide solution andforming sulfur reduced sodium polysulfide, employing at least a portionof said sulfur reduced sodium polysulfide as an electrolyte in anelectrolytic cell for the production of sodium metal, and, thereafter,contacting at least a portion of said sodium metal withsulfur-containing petroleum oil stock.

7. The process of claim 6 wherein said oil stock is selected from thegroup consisting of whole crude and residua.

8. The process of claim 6 wherein said oil/sodium sulfide mixture iscontacted with water at a pressure ranging from about 330 to 540p.s.i.g. and at a temperature ranging from about 425 to 475 F. therebyforming an aqueous sodium sulfide solution and an oil of reduced sulfurcontent.

9. The process of claim 8 wherein said oil of reduced sulfur content isfurther contacted with water at a temperature ranging from about to 205F. and at a pressure ranging from about 5 to 10 p.s.i.g.

10. The process of claim 6 wherein said sodium polysulfide isrepresented by the formula Na S where x takes values from about 4.4 to4.9, and said sulfur reduced sodium polysulfide is represented by theformula Na s where y takes values ranging from about 3.5 to 4.3.

11. The process of claim 6 wherein said electrolytic cell comprises ananodic cavity containing polysulfide anions and a cathodic cavitycontaining sodium cations, said anodic and cathodic cavities separatedby a sodium ion-conducting membrane.

12. The process of claim 11 wherein said membrane comprisesbeta-alumina.

13. The process of claim 6 wherein said aqueous sodium sulfide solutionis contacted with calcium hydroxide and hydrogen sulfide prior tocontact with said sodium polysulfide.

14. -In a process for the desulfurization of a sulfurcontainingpetroleum oil stock comprising contacting said oil stock in adesulfurization zone with a desulfurization agent selected from thegroup consisting of sodium and alloys thereof, thereby precipitating atleast a portion of the sulfur contained in said oil stock as sodiumsulfide and forming an oil/sodium sulfide mixture, the improvemeritcomprising contacting said mixture with water at a pressure ranging fromabout 250 to 700 p.s.i.g. and at a temperature ranging from about 400 to500 F., thereby forming an aqueous sodium sulfide solution and an oil ofreduced sulfur content, contacting said oil of reduced sulfur contentwith additional water at a temperature ranging from about 150 to 300 F.and at a pressure ranging from about 0 to 75 p.s.i.g., and recovering adesulfurized petroleum oil stock.

15. A process for the desulfurization of a sulfur-containing petroleumoil stock comprising introducing said oil stock into a desulfurizationzone, contacting said oil stock therein, at desulfurization conditions,with a desulfurization agent selected from the group consisting of thealkali metals and alloys thereof, thereby precipitating at least aportion of the sulfur contained in said oil stock as alkali metalsulfide and forming an oil/alkali metal sulfide mixture, contacting atleast a portion of said mixture with water at a pressure ranging fromabout 250 to 700 p.s.i.g. and at a temperature ranging from about 400 to500 F. thereby forming an aqueous alkali. metal sulfide solution and anoil of reduced sulfur content, contacting said oil of reduced sulfurcontent with additional water at a pressure ranging from about 0 to 70p.s.ig. and at a temperature ranging from about 150 to 300 F., therebyremoving residual alkali metal sulfide from said oil, and recovering adesulfurized oil.

(References on following page) References Cited UNITED STATES PATENTSCo'bb 208-229 Cobb 208-230 Kaneko 208-208 5 Kimberlin, Jr. et a1.

208-208 M Haskett 208-208 M Kaneko et a1 208-208 Schulze et a1 20s 230Mattox 208-230 1 6 Kummer 204-180 Kurnmer 13 6-6 Kummer 136-6Tennenhouse 13 6-6 Tennenhouse 136-6 DELBERT E. GANTZ, Primary ExaminerI. M. NELSON, Assistant Examiner US. Cl. X.R.

